Interpretive Summary: Tropospheric ozone is a major air pollutant that has adverse effects on the growth and yield of agricultural crops. The development of ozone tolerant plants is one approach to alleviate this problem. Improvement of ozone tolerance requires knowledge of the critical points in plant metabolism that can be manipulated to provide greater protection against ozone stress without sacrificing yield or other desirable characteristics. Ascorbic acid (vitamin C), a compound synthesized in plants, has the potential to neutralize ozone in leaf tissue after the gaseous pollutant enters via the leaf stomata. In this study, leaves from ozone-sensitive and ozone- tolerant snap bean genotypes were analyzed for ascorbic acid to determine if ozone sensitivity was related to differences in this antioxidant vitamin. Although leaf ascorbic acid content was similar in all genotypes, ozone sensitivity was related to the cellular location of ascorbic acid. Ozone-sensitive genotypes initially contained low levels of ascorbic acid in the external fluid surrounding leaf cells and then responded to ozone injury by increasing the amount of ascorbic acid in the extracellular space. In contrast, a tolerant genotype had much higher levels of ascorbic acid in the external fluid prior to ozone exposure, and thus may have a greater capacity to neutralize ozone and prevent reactions that lead to cellular injury. The results suggest that one potential tolerance mechanism is the capacity to transport ascorbic acid between the cytoplasm and extracellular space of leaf cells. Additional research is needed to confirm and expand on this

Technical Abstract:
The effect of ozone on the partitioning of ascorbic acid between the apoplast and cytoplasm of plant cells was investigated in three cultivars (Oregon 91, Strike, and Tenderette) and one ozone- sensitive genotype (S248) of snap bean. Plants were grown in pots of soil under field conditions during the months of June and July. Open top chambers were used to establish either a charcoal filtered (CF) air control (36 nmol mol-1 ozone) or a treatment where CF air was supplemented with ozone from 08:00 to 20:00 h with a daily 12 h mean of 77 nmol mol-1. Fully expanded trifoliate leaves were compared during a 7-day ozone exposure. Chlorophyll content and guaiacol peroxidase activity were used to assess leaf injury. Vacuum infiltration methods were employed to separate leaf ascorbic acid into apoplast and cytoplasm fractions. Sensitive genotypes initially partitioned a relatively low percentage (~ 1%) of total ascorbic acid into the apoplast. Ozone treatment induced an increase in apoplast ascorbic acid content that was proportional to the ozone sensitivity of the plant. A maximum of 7% of total ascorbic acid was partitioned into the apoplast in the most sensitive genotype S248 by the end of the 7-day ozone exposure. In contrast, the most ozone-tolerant genotype Tenderette partitioned 3-4% of total leaf ascorbic acid into the leaf apoplast in both control and ozone treated plants. The results suggest that ascorbic acid in the cell wall and/or extracellular space plays a role in plant response to ozone stress and may be one factor that determines ozone tolerance. Measurements of ascorbic acid redox